1. Field of the Invention
The present invention is directed to technology and design of ultrasound transducer arrays with symmetric electronic steering of the focus for ultrasound imaging, particularly both two-dimensional and three-dimensional medical ultrasound imaging.
2. Description of the Related Art
Ultrasound array transducers are used in ultrasound imaging for electronic direction steering and focusing of the ultrasound beam. The commonly used arrays have a linear arrangement of the elements for two-dimensional scanning of the beam. The linear phased arrays, for example, produce a sector scanning of the beam centered at the array, while the linear or curvilinear switched arrays provides a wider image field at the transducer.
A problem with the linear arrangement of the elements, is that the beam focus can be electronically steered only within the two-dimensional (2D) scan plane, what is referred to as the azimuth direction. The beam focus in the direction normal to the 2D scan plane, what is referred to as the elevation direction, must with these arrays be set to a fixed depth.
In many practical situations one makes a 2D ultrasound image where the variation of the object is limited transverse to the 2D scan plane (i.e. in the elevation direction). Such examples are short and long axis imaging of the heart, imaging of the fetal trunk and head, amongst other. In such cases there is limited need for electronic steering of the elevation focus. On the other hand, imaging of objects with short dimension in the elevation direction, like vessels, cysts, a fetal heart, etc., is greatly improved when the beam has an electronically steered focus both in the elevation and the azimuth directions. Electronic steering of both the elevation and azimuth focus is also important for three-dimensional (3D) imaging where the object can be viewed from any perspective (direction) that favors optimal focusing with minimal resolution in all directions.
Electronic steering of the focus in the elevation direction can be obtained by dividing the linear array elements into sub elements in the elevation direction. A particular solution to such steering of the elevation focus is given in U.S. Pat. No. 5,922,962. However, to obtain full symmetric steering of the azimuth and elevation foci, a large number of elements is required with this solution, complicating the cabling and drive electronics for this array. Also, the elements of this array becomes small, increasing the electrical impedance of the elements that increases noise and cable losses, which further limits the maximal frequency that can be used with such arrays for a given depth, and consequently the resolution obtainable with these arrays at a given depth.
Another, well known method to obtain an electronically steered symmetric focus is to use an array of concentric annular elements, the so-called annular array. Such an array is usually pre-focused mechanically to a depth F, either by curving the array or by a lens, or by a combination of the two. The focus, F, is then steered electronically from a near focus Fn less than F to a far focus Ff greater than F by adding delays to the element signals before they are added, according to well known principles. The beam will then be optimally focused symmetrically around the beam axis, i.e. equally focused in the azimuth and the elevation directions, with fewer and larger elements than with the 2D arrays described above. This gives lower electric impedance of the elements, reducing noise and cable losses with improved sensitivity compared to the 2D arrays. For mechanical scanning of the beam direction, the annular array is immersed in a fluid inside a dome. The array itself is therefore not pushed against the skin as the linear arrays, and can hence be made with a lighter weight backing than the linear arrays, for example a plastic foam. This reduces the backing losses which further improves the sensitivity of the annular arrays above the linear 2D arrays. The improved sensitivity of the annular array hence allows the use of higher ultrasound frequencies, which further improves the image resolution above the linear 2D arrays.
The fewer number of elements of the annular array compared to the 2D array, allows the use of wider apertures, which further reduces the focal diameter, and hence improves the lateral resolution. With very wide aperture annular arrays, however, the outer elements can become quite narrow when steering of the focus over a large range is required. This can introduce complex vibration modes of the elements, reducing the efficiency of the elements. Further, narrow elements complicate the manufacturing and increase the total number of elements in the array which complicates electrical connections to the moving array.
The present invention presents a solution to this problem with annular arrays by acoustically pre-focusing the annular elements at different depths, where a core group of elements are pre-focused to participate in the active aperture for the whole image range. Outer elements that are pre-focused at deeper ranges are then included to the active aperture at deeper ranges so that the angular expansion of the focal diameter with depth is reduced by the increased aperture size. The invention hence allows the full use of the advantages of the annular arrays: 1) A symmetrical focus that is steered electronically within the actual image range, 2) fewer and larger elements with the annular array with lower impedance backing gives high sensitivity that allows for the use of high frequencies with improved resolution, and 3) the lower number of elements simplifies the front end electronics.
Objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.